Determination of Organosulfides from Onion

foods Article Determination of Organosulfides from Onion Oil Maranda S. Cantrell 1,2, Jared T. Seale 2, Sergio A. Arispe 3 and Owen M. McDougal 2,* 1 Biomolecular Sciences Ph.D. Program, Boise State University, Boise, ID 83725, USA; [email protected] 2 Department of Chemistry and Biochemistry, Boise State University, Boise, ID 83725, USA; [email protected] 3 Malheur County Extension Office, Oregon State University, Ontario, OR 97914, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-208-426-3964 Received: 13 June 2020; Accepted: 2 July 2020; Published: 6 July 2020 Abstract: Qualitative and semi-quantitative analysis of organosulfides extracted from oil obtained by steam distillation of yellow onions was performed by gas chromatography-mass spectrometry (GC-MS). The extraction efficiency of organosulfides from onion oil was evaluated across four solvents: dichloromethane; diethyl ether; n-pentane; and hexanes. Analysis of solvent extracted organosulfides by GC-MS provided qualitative results that support the use of dichloromethane over other solvents based on identification of 27 organosulfides from the dichloromethane extract as compared to 10 from diethyl ether; 19 from n-pentane; and 17 from hexanes. Semi-quantitative evaluation of organosulfides present in the dichloromethane extract was performed using diallyl disulfide as the internal reference standard. Three organosulfides were detected in the extract at 5 mg/kg; 18 organosulfides between 3–5 mg/kg; and six organosulfides at <3 mg/kg. The E/Z ≥ isomers of 1-propenyl propyl trisulfide were among the most prevalent components extracted from the onion oil across all solvents; and 3,6-diethyl-1,2,4,5-tetrathiane was among the most abundant organosulfides in all solvents except hexanes. The method described here for the extraction of organosulfides from steam distilled onion oil surveys common solvents to arrive at a qualitative and semi-quantitative method of analysis for agricultural products involving onions; onion oil; and secondary metabolites of Allium spp. Keywords: yellow onion; onion oil; organosulfide; food flavoring; Allium cepa L. 1. Introduction Steam distilled onion oil is popular in the food and health industry because organosulfide components of the oil infuse succulent flavors, serve as antioxidants, suppress premature food browning, and delay the propagation of pathogenic and spoilage microorganisms [1]. One commercial method to obtain onion oil from yellow onions is by steam distillation; the onion oil is sold as a commodity to restaurants as a food flavoring ingredient, or as a therapeutic essential oil for hair follicle revitalization and homeopathic remedy for alopecia areata [2,3]. The methods for extraction of organosulfide components from steam distilled onion oil have involved the use of solid phase microextraction, headspace, or liquid–liquid extraction [4–6]. Solid phase microextraction involves the use of specialized fibers coated with extraction polymer or adsorptive particles embedded in a polymer that capture volatile molecules for analysis by gas chromatography-mass spectrometry (GC-MS) and has been recently used to compare the composition of the volatile organic compounds of onion and shallot [7]. The solid phase microextraction fibers are specific for volatile organic compound analysis, making selection of the correct fiber essential for optimal component analysis. Sample preparation to ensure sufficient volatile organic compound Foods 2020, 9, 884; doi:10.3390/foods9070884 www.mdpi.com/journal/foods Foods 2020, 9, 884 2 of 13 adsorbance onto the solid phase microextraction support to permit GC-MS injection concentrations suitable for analyte detection and characterization requires stringent method development. The solid phase microextraction fiber sample holder, solid phase microextraction fibers, and headspace injection system adds to the initial cost to implement this technique. Headspace analysis of volatile organic compounds also requires the headspace assembly accessory for the GC. The volatile organic compound concentration can be a challenge as gas phase injection into the GC is inherently dilute [8,9]. Lastly, the headspace volatilization of sample, like onion oil, requires heating, which can degrade the volatile organic compounds prior to their identification [9]. Perhaps the most rudimentary and reliable process for volatile organic compound characterization from onion oil is liquid–liquid extraction. In the current study, a series of organic solvents were evaluated for qualitative and quantitative extraction of organosulfides from onion oil obtained by steam distillation of yellow onions (Allium cepa L.), based on the method of Tocmo et al. [10]. The four organic solvents assessed for liquid–liquid extraction of organosulfides from onion oil were dichloromethane (DCM), diethyl ether (DEE), n-pentane, and hexanes. Selection of these solvents was based on literature precedent in studies of the anticancer and antibacterial properties associated with the organosulfide constituents of garlic (Allium sativum L.) and onions (Allium cepa L.) [11–14]. The solvents most commonly reported for organosulfide extraction from Allium oils are DCM and DEE [10,15–17]. n-Pentane and hexanes are less commonly reported solvents for extraction of organosulfides, but there are studies that have demonstrated their effectiveness with garlic and other Allium spp [18,19]. The organosulfide profile of onions has been widely studied, along with the effects on organosulfide formation upon treatment of onions to conditions including boiling, freezing, freeze drying, and even different methods of homogenization in various solutions [4,12,20–22]. However, the extraction solvent used for onion oil from yellow onion has been little studied. Here, we report a comparison of four commonly used organic solvents to extract organosulfides in onion oil derived from steam distilled yellow onions to determine which solvent provides superior qualitative and semi-quantitative results upon analysis by GC-MS. While qualitative identification of organosulfide constituents in onion oil has been performed, quantitative yields correlated to extraction solvent have not been reported [4,6]. An article describing the optimization of GC methods for compound identification of volatile organic compounds extracted from raw onion has also been published, but the study did not address quantitative analysis on steam distilled onion oil [19]. In the present study, we sought to determine the optimal extraction solvent for steam distilled onion oil to provide the best qualitative and semi-quantitative characterization of organosulfides present in the oil. The term semi-quantitative is used to refer to organosulfide quantitation using a single calibration standard, diallyl disulfide, to approximate the concentration of all organosulfide constituents in each extract. It is envisioned that food processors seeking to implement quality control measures on onion oil food flavoring products may adopt the method presented here. 2. Materials and Methods 2.1. Chemicals and Materials Dimethylchloride (DCM; HPLC grade > 99%), diethyl ether (DEE; anhydrous, HPLC grade > 99%), n-pentane (anhydrous, >99%), hexanes (>98.5%), and diallyl disulfide (DADS, >80%) were purchased from Fisher Scientific (Hampton, NH, U.S.A.). Polytetrafluoroethylene (PFTE; 0.45 µm) filters were also purchased from Fischer Scientific. 2.2. Sample Preparation Yellow onions of species Allium cepa L. were purchased from a local supermarket and kept at room temperature until use. The onions were prepared by pealing their skins, removing the ends, and rinsing with 18 megaohm (MW) nanopure water. Approximately 155 g of freshly peeled onion was homogenized using a Hamilton Beach 10-cup Food Processor Model 70,730 with enough nanopure Foods 2020, 9, 884 3 of 13 water to cover the onions. The homogenate was placed in a 500 mL round bottom flask and steam distilled for 3.5 h. The distillation process was repeated with fresh onion until approximately 500 mL of steam distilled onion oil was obtained. From this 500 mL stock of onion oil, approximately 35 mL of the milky distillate was transferred to a separatory funnel, and combined with equivalent amounts of extraction solvent, either DCM, DEE, n-pentane, or hexanes. The separatory funnel was cocked and shaken with intermittent pressure release until the buildup of gas no longer occurred. The organic layer was collected, and the milky aqueous layer underwent two additional extractions of 35 mL solvent each, until the aqueous layer appeared clear. The organic extracts were combined, and the solvent evaporated overnight in a chemical fume hood, leaving the flask with the desired yellow–green onion oil residue. The oil was weighed and dissolved in 1 mL DCM, then filtered through a pre-wetted 0.45 µm PFTE membrane, spiked with 2 µL of a 1% (v/v) solution of DADS in DCM as an internal standard, and analyzed by GC-MS. 2.3. GC-MS Method & Compound Identification The GC method, adapted from Tocmo et al., was performed using a Thermofisher Trace GC Ultra system equipped with a fused silica TG-5MS (30 m 0.25 mm i.d., 0.25 µm) column [7]. The injection × port temperature was 250 ◦C, and the run method began with an initial temperature held at 50 ◦C for 3 min, raised at 3 ◦C/min to 150 ◦C, held at 150 ◦C for 3 min, and finally ramped at 25 ◦C/min to a final temperature of 250 ◦C, where the temperature was held for 5 min. Helium was the carrier gas with a flow rate of 1 mL/min. One µL of sample

Load more